Barriers and facilitators for recruiting and retaining male participants into longitudinal health research: a systematic review

Background Successfully recruiting male participants to complete a healthcare related study is important for healthcare study completion and to advance our clinical knowledgebase. To date, most research studies have examined the barriers and facilitators of female participants in longitudinal healthcare-related studies with limited information available about the needs of males in longitudinal research. This systematic review examines the unique barriers and facilitators to male recruitment across longitudinal healthcare-related research studies. Methods Following PRIMSA guidelines, MEDLINE, Embase, CINAHL and Web of Science databases were systematically searched using the terms recruitment and/or retention, facilitators and/or barriers and longitudinal studies from 1900 to 2023 which contained separate data on males aged 17–59 years. Health studies or interventions were defined longitudinal if they were greater than or equal to 12 weeks in duration with 3 separate data collection visits. Results Twenty-four articles published from 1976–2023 met the criteria. One-third of the studies had a predominantly male sample and four studies recruited only male participants. Males appear disinterested towards participation in health research, however this lack of enthusiasm can be overcome by clear, non-directive communication, and studies that support the participants interests. Facilitating factors are diverse and may require substantial time from research teams. Conclusions Future research should focus on the specific impact of these factors across the spectrum of longitudinal health-related studies. Based on the findings of this systematic review, researchers from longitudinal health-related clinical trials are encouraged to consider male-specific recruitment strategies to ensure successful recruitment and retention in their studies. Registration This systemic review is registered with the PROSPERO database (CRD42021254696). Supplementary Information The online version contains supplementary material available at 10.1186/s12874-024-02163-z.


Background
Recruitment into and continued participation of participants in clinical research provides continued challenges for researchers [1].This is particularly true for participants who identify as men, likely due to the social roles and norms gender plays in society [2].Recruitment is time-consuming, expensive, and the involvement and retention of male participants, as part of longitudinal healthcare studies, can be enormously demanding.It is likely that social constructs related to men such as cultural perceptions and health-seeking behaviour [3], have contributed to the challenges of male participant recruitment in healthcare-related research.However, these specific barriers have not been systematically investigated as part of previous clinical-research studies.Previous studies have explored the attitudes, beliefs and knowledge of the public towards research and research participation, focusing on clinical trials [4].The public's willingness to participate may be informed by a favourable attitude towards researchers, comprehension of the trial rationale, or the specific clinical circumstances (e.g., having a non-fatal disease with no known cure, being healthy, or critically ill with a limited chance of survival) [4].It is important to note that, research findings often require a considerable amount of time to transition into clinical practice, and it is essential to educate the public about this process to encourage their participation in studies, ultimately advancing the progress of research.Given the time lag between findings resulting from healthcare research studies to healthcare implementation, an important component of participation is to enhance the public understanding of healthcare related research studies [1,4,5].
There are a range of population groups among whom it can be particularly challenging to recruit as part of longitudinal health studies and can include disadvantaged, minority and vulnerable members of the community.While others have systematically reviewed the recruitment and retention of participants in health studies related to conditions including cancer, dementia, and HIV, as well as studies involving vulnerable populations [6][7][8], less is known about recruitment and retention of male participants as part of longitudinal health-related studies.This highlights the need to address recruitment issues in a broad spectrum of healthcare-related research studies for males.
Although several healthcare-related studies have examined recruitment of male participants across diverse populations groups, there is limited research identifying barriers and facilitators associated with overall male recruitment into healthcare-related studies.Notably, there are male-specific clinical changes across healthcare that can influence interest in related research [9].Life expectancy is lower in males [3], especially those aged over 50 years, who often experience a greater disease burden [10].Although previous studies demonstrate that men are disengaged with healthcare services, it is now recognised that males engage willingly and effectively with healthcare that recognise the preferences of males [10][11][12].Previous literature have investigated methods of improving male recruitment to health behaviour research [13].Indeed, sex was an important determinant of health-risk and health-promoting behaviours [14], with males being more likely to perform high-risk behaviours including smoking, unhealthy eating, excess alcohol consumption, and physical inactivity [3,15] and despite this, remained less likely to seek medical and psychological help when needed [16] or to participate in health-promotion programs [17].Maher et al., detailed that males only comprise about 20% of health behaviour research participants, in mixed sex studies [18], contributing to a lack of evidence on how to increase the uptake of health-promoting behaviours for males [19].These findings highlight the need for highly effective, male-specific methods to assist recruitment and retention in research studies in line with current best practice and guidelines.
Effective long-term recruitment methods to enable and facilitate male recruitment in longitudinal healthcare research consistently demonstrate strategies should be tailored for age, interests, and sex.To facilitate the effective recruitment of men into research, different recruitment methods for different age groups of either sex can be effective [11,20].Younger males may be recruited through online social network platforms including Facebook [21,22], which is less effective in elderly males [23].While elderly men would be more likely to participate if referred to the study by their affiliated health service provider, media coverage or mass mailings [11].Facebook, in particular, is more effective at recruiting participants than any of the other social media platforms combined [21].As of October 2020, more males globally (57%), use Facebook than females (43%) [24].Yet, a recent systematic review of recruitment using Facebook, found little evidence of its effectiveness in recruiting participants of either sex aged over 35 years [22], highlighting that social media strategies were ineffective.Tolmie et al., reported that the need for ongoing health monitoring for older participants was the most important recruitment and retention motivator, in addition to fostering positive relationships between staff and participants, and communicating the studies progress to recruits [25].
The difficulty of recruiting and retaining males in research studies can adversely affect the statistical study power and generalisability of study findings, and in particular, those studies involving a longitudinal design which consequently affects the applicability of results to the male population [26,27].While sex (male) and gender (men) constructs are important considerations in society and within health, to date, the terms male and men are often used interchangeably in health literature.For these reasons, this systematic review has reviewed published studies that consider male participation, recognising that the terms male and men, most often refer to the biological construct of male sex.This review has focused on health research or health interventions that are using a longitudinal study design.The main outcome was to identify specific barriers and facilitators of male recruitment and retention as part of longitudinal research-related studies.Findings have the potential to inform future development of patient-centred and evidence-based strategies to enhance recruitment into longitudinal health-related studies for men.

Methods
This systematic review protocol was registered with PROSPERO database (University of York Centre for Reviews and Dissemination) (CRD42021254696) and complies with reporting guidelines from the Preferred Reporting Items for Systematic Reviews and Meta-analysis (PRISMA) statement [28].

Study identification
Studies published in English without date restrictions were identified through systematic searching.There was no limit applied to the dates of publications, in order to explore the full breadth of the literature surrounding the topic and determine strategies used that remain relevant in this current time.The databases MEDLINE (Ovid, 1946 to present), Embase (Embase.com,1947 to present), CINAHL (EBSCO, 1981 to present), and Web of Science (Clarivate Analytics, 1900 to present) were searched on 20 October 2020.The searches were updated on 21 October 2021 and 11 November 2023 to determine any additional publications during the 2020-2023 period.The MEDLINE search strategy was translated for the other databases using the Polyglot Search Translator [29].Specific search terms used (see Supplementary File 1) included recruitment and/or retention, facilitators and/ or barriers, and longitudinal studies.Here, longitudinal research was defined as a study with a minimum of three repeated study visits, or research data collections over a time greater than or equal to 12 weeks.Search terms were used with Boolean operators "AND" and "OR" as connective devices within the search strings.Where appropriate and possible, search terms were truncated (*) to retrieve multiple variations of a word.
All retrieved articles, excluding duplicates, were exported into Covidence [30] to facilitate the screening process.Identified studies were screened by two independent reviewers from the review team (DJB, MH-S, PG, HWT, DH, DS) to identify eligible studies.Studies were assessed for inclusion based on screening of title and abstracts.A third independent reviewer (KMR) reviewed conflicts.Full text papers were retrieved and assessed by two team members according to the inclusion and exclusion criteria with conflicts reviewed by third independent reviewer (KMR).The reference lists and citing articles of included studies and relevant reviews and systematic reviews were further hand-searched for further potential papers for inclusion.

Types of participants
This review includes any participant who identifies as male (biological sex).Where the included publication does not make it clear if this is the biological definition of males, or the gendered definition of men, the assumption has been made that this refers to those who are biologically male, and therefore included.Male participants between the ages of 17 -59 years were included.We chose 17 years of age as the lower age limit to encompass studies involving adult males that did not require parental consent.The upper limit of 59 years was set to eliminate recruitment of older populations, as previous research has covered participation of older populations extensively and we aim to investigate factors influencing the involvement of younger individuals.Studies covering a broader age range were considered only if they provided age-specific data.Male parents, who were consenting on behalf of their child into a longitudinal health study or intervention were included.Parents who consent on behalf of a child are often needed to take part in certain aspects of the study; however only studies that specifically identified parental sex were included.Studies that included females or indeterminate sex were reviewed however these studies were only included if recruitment and/or retention of males and men were reported separately.Likewise, studies of parents and child or family studies were included if they reported recruitment and/ or retention of male parents separately.

Types of studies
Any cross-sectional, longitudinal, survey, experimental, program evaluation studies or studies involving qualitative or mixed methods that intentionally (i.e., stated a priori) or incidentally (i.e., noted as a posteriori) included detailed commentary or analysis on the recruitment and/ or retention of male participants in a longitudinal health intervention or health research study, with the requirement that this commentary offered informative data rather than a generalised statement about participant recruitment or retention.

Types of exposures/interventions
This review excluded studies focused on Alzheimer's/ dementia [31], cancer [32], HIV [33] and illegal drugs [34] due to the wealth of existing systematic review literature.Studies focusing on fathers with young children in early childhood health intervention research were excluded due to a recent systematic review [35].
Any other health research program or health intervention was included, provided enrolled male participants had data collected on a minimum of three separate occasions over a period of greater than or equal to 12 weeks.Longitudinal studies that were less than 12 weeks in duration or had less than 3 study visits or data collections were excluded.A health intervention was defined as any study aimed at improving specific health behaviours or outcomes.

Types of outcome measures
Studies were included if they identified specific strategies for recruiting and retaining male participants into longitudinal research and longitudinal clinical practice and if findings were analysed, reported, or discussed separately.

Study population
Studies on recruitment, retention, barriers and facilitators of vulnerable populations, males < 16 years and males > 60 years of age were excluded.Vulnerable male populations were defined as socioeconomically disadvantaged populations or racial and ethnic minorities (including Indigenous and First Nations people).Due to the cultural, economic and other differences that a review of these communities would likely identify, it was deemed to be appropriate for these populations to be reviewed separately in the future.

Study topic
Conference abstracts, review or systematic review papers, incomplete studies including study protocols, and grey literature were excluded from this review.Articles were excluded at any time in the screening process, if they did not (1) examine views or include discussions that considered retention, barriers, or facilitators for research/interventions; (2) include male specific data, and only discussed female participants; (3) determine the participant sex or (4) focus on participant recruitment or retention as part of the health research/intervention.

Data extraction
Data extraction was performed by three members of the review team (DJB, PG, MH-S) and reported narratively.Extraction was cross-checked for accuracy and consistency by one other reviewer (either DS or KMR).The following information was extracted from each included study: publication information, study aim, methods (i.e., participants, procedures, demographics), recruitment of male participants, retention of male participants, timing of data collection, and types of data collected from male participants.Reported barriers and facilitators to support male recruitment and retention was extracted.

Quality assessment
A quality assessment check is usually undertaken in a systematic review that pertains to a review that assesses the individual results of a group of specific studies.As this review assesses the barriers and facilitators to recruitment and/or retention methods, there was no need for a quality check of the included studies.

Results
The database searching and the forward and backward citation checking yielded 16,457 and 13 papers respectively (16,470 total).6,108 duplicates were removed resulting in 10,362 articles available for screening (Fig. 1).Of these, 9,214 studies did not meet the inclusion criteria based on titles and abstract screening and resulted in 1,148 full-text studies selected for further screening (Fig. 1).A total of 1,124 studies were then excluded with 255 having no male specific data, 166 conference abstracts, 115 HIV related research, 106 cancer related research, 78 studies had no included data on barriers or facilitators, 71 studies with a focus on males > 60 years, 69 studies from racial or ethnic minority, 52 studies were unrelated to health recruitment and retention, 48 Alzheimer's or dementia research, 39 related to illegal drugs, 29 papers were studies with less than 3 study visits, 24 papers were males < 16 years of age, 22 systematic review/review papers, 19 focused on socioeconomically disadvantaged populations, 14 uncompleted studies/ study protocol, 13studies were < 12 weeks duration, and 4 fathers in early childhood interventions (Fig. 1).
A total of 24 articles remained and the data was extracted and included in this review.The oldest of these studies was published from 1976 [36] and the most recent, 2023 [37,38].All of the included studies were conducted in Western countries except Cheraghi et al., which was based in the Middle East [39] and Schilling et al., which was based in India [37]; two were located in United Kingdom [40,41], two in France [42,43], one in Finland [44], one in Sweden [45], one in The Netherlands [46], one study across combined European nations [29], one in Germany [47] ten in North America [36,[48][49][50][51][52][53][54][55][56] and three in Australia [38,57,58] and are described in Table 1.Participant characteristics varied with study focus including participants with specific health conditions, such as overweight [41,57], having an occupational injury [40,41], having visited a sexually transmitted infection clinic [46], or being treated for a psychological disorder [44,50,53], COVID related issues [37,54], or habits such as alcohol abuse and smoking [56].Some studies recruited participants from specific subgroups, including veterans [36], workers of an electricity company [42] and people that had attended a spouse abuse abatement program [50].All twenty-four studies met the inclusion criteria for age.One of the studies was a family cohort study that recruited families of children with cystic fibrosis and congenital heart disease and required participation of both parents [51].
Of the included studies, 20 had male and female participants [37-40, 42-49, 51-58], with a number of these  studies having a predominantly male sample [42,52,53,58].Four studies recruited only male participants [36,41,50,61] (Table 2).The included studies with mixed sex either described male and female characteristics separately or clearly stated that there were no significant differences in recruitment and retention based on sex.All included studies used a minimum of three study visits or data collection, and the maximum number of study visits or data collections was 95 visits [41] and one study had up to 300 interactions with participants [44].The minimum study length of included studies was 16 weeks [50] and the maximum study duration was 43 years [45].All included studies collected demographic data 61].

Recruitment
Overall, all studies provided information on recruitment rates and 19 provided information on retention rates [36, 38-41, 43-47, 49-52, 55-58, 61] (Table 2).A variety of methods for male participant recruitment included advertising [36,43,54,57], letters of invitation [39-43, 47, 52, 56-58, 61], selection of participants from larger cohorts [42,43,50,53], or recruitment from hospitals or registers [37,44,48,51,54,57] (Table 3).The most common method was sending letters of invitation, used in 11 out of the 24 studies, and yielded recruitment rates between 4.4% and 79.3% [47,52].Irvine et al., recruited participants through letter of invitation and time space sampling, and reported that time space sampling was difficult, time consuming and only yielded one participant per 11 field visits [23].Snow et al., used multiple methods for recruitment, including recruitment from work sites and public sites, mass mailing, telephone, media, and referral methods and reported that mass mailing was the best method of these [55].Rose et al., attributed their high recruitment rates to advertising and therefore people that agreed to participate had done so voluntarily and were more likely to be interested in the study and health interventions in general [36].To maximise male participation, vanWees et al., adapted their recruitment methods to target male participants by raising awareness and a greater sense of responsibility in terms of male health through flyers or personalised invitations [46].

Barriers
A variety of factors were identified that interfered with male participation in longitudinal research are shown in Table 4.Some of these were situational and included participant death or relocation [36,39,42,45,48,51,53,55,57,61].While other barriers included time commitment [40,58], reluctance for medical testing [58], or the belief that the study is an invasion of privacy [58].A large number of studies reported that men did not attend study visits [40,58], were not interested in the study or could not be bothered to participate [36,40,41,44,48,49,51,58,61], and study staff received no response to invitations [40,41,61].

Discussion
We have undertaken a thorough assessment of longitudinal studies to determine recruitment and retention facilitating strategies for male participants.Retention of male participants was particularly impressive in two studies at 85.5% (from n = 69 recruited) [41] and 88.9% (from n = 2,280 recruited) [36] over a period of 5 months and 12 years, respectively.Irvine et al., undertook a trial to reduce alcohol related consumption to reduce obesity and relied upon the perceived health benefits for their participants [41], while Rose et al., studied ageing in Veteran participants.Rose et al., retained participants over 12 years, through a diverse range of approaches including; the use of participant newsletters, short study visits, free medical screening, income supplementation, encouraged participant perceptions of being part of the 'health elite' and recruited wives to assist with retention [36].This study began in 1976 where there was a greater inclination by the public to follow suggestions, also particularly true for their target Veteran population [36].Like Rose et al. [36], the Irvine et al., study team maintained regular contact, ensuring convenient timing and location of study visits and continued to highlight perceived health benefits of the research project.The Irvine et al., research team spent additional time ensuring that their staff conversations and project literature was relaxed and friendly and non-directive in its approach [41].While Rose et al., provided re-imbursement to employers for study attendance, neither of these top two studies [41] used a direct financial or gift incentive to participants but rather relied upon excellent communication strategies.Several studies highlight that different aged males are retained at different rates in their studies.For example, Cheraghi et al., saw 67% 40-59 year males retained while only 55% of > 60 years in the same study were retained [39].In the male only studies, Hamberger et al. [50], and Lee et al. [61], showed varied retention rates based upon age of the participants.Male only studies have shown that diverse approaches can be successful in recruitment and retention.Communication that is non-directive in style, clear and delivered by supportive staff was important for Irvine et al. [41].Continuing to maintain contact with male participants was important and included contact through family or a spouse [36], Christmas [61] and birthday cards, and study related newsletters [36].Male only studies have highlighted that where male participants have a vested interest, for example, weight loss and desire for health education, these interests can be important drivers for recruitment [41].

Barriers to recruitment and retention of males
Barriers varied and were related to an inability of participants to participate due to lack of understanding of study objectives [58], language barriers [58] or lack of access to the internet for studies being conducted online [53].Table 4 highlights reasons given for refusal to participate and reasons for non-completion of a study.
In intervention studies focused on lifestyle changes, barriers to participation included inability to adhere to the study activities [57] or lack of motivation to engage with new technologies [44].In one study where participants had to make dietary changes and frequently visit the research centre, participants expressed frustration in trying to implement study content due to associated financial costs involving more expensive food, transportation or computer access [53].Participant feedback included that transportation and free meal options would have been more enticing [44].Travelling to the study centre was found to be a barrier in two studies [53] and another study participants expressed a preference for study to be online [53].Another barrier reported in two studies was difficulty in arranging a follow-up session [53].
The major causes for refusal or dropout were time commitment issues and lack of interest in the study [40,44,51,58].Time commitment issues were related refusal to having to make frequent visits to the study centre [51,57,58] and lifestyle changes that required more time, such as exercising or taking time to cook meals [53].Health issues played a role in participant attrition and participants with health issues [42,51,53,57,58] and psychological issues [50] were most likely to refuse participation or dropout.
Different demographic characteristics were associated with refusal to join a study or a particular data collection point.These included low socioeconomic status [40,42], younger age [40,[42][43][44]52], older age [40,61], poor lifestyle factors [42] and being unmarried [43,45].In a birth # Odds ratio reported only for male participants retained [39] or division of age groups [59] * Data not divided between those identifying as men and those who identify as women a Study protocol of PREVENT ADPKD [63], substudy visits not specified [38] but is assumed 2 (questionnaire, interview) in a 16 month period b [40] and study protocol [60].Study launched reported on study website [64] c Protocol paper describes 3 Stages after recruiting a population-based cohort [60], not described in [40] d Study contact were text messages the amount, timing and frequency of SMS text messages were decided by participant [44] e Completers of final postal survey f Last wave (wave 5) of the study (77 -87 years old) completers of follow-up, no male specific data given for wave 1-4 [45] g Protocol paper describes the use of census data at 1996, 2001 and 2006 [65] h Yearly visits with an option to fill in a complementary questionnaire each month [43] i Two studies were included WebMOVE and MOVE-SMI [53] j Sub-study was randomly selected from WebMOVE and MOVE-SMI groups and not stratified by sex [53] k Details only recruitment strategies [54] l Due to its longitudinal nature, it is assumed this study has more than 3 visits in the 12-year reported period.No information on number of study visits or contacts was found in Rose et al. [36] or the associated protocol [66]

Reasons for refusal to participate or drop-out Proportion of participants who refuse participation References
Appointment non-attendance 14.0% of passive refusals [40] 2.1% of males that refused to participate [58] 1.3% of males were unable to cope with study requirements due to old age [58] Comprehension of the study requirements 0.4% of males refused to participate [58] Cannot be bothered/not interested 27.8% of active refusals [40] 13.8% declined to participate [41] 66.7% refused to participate [44] 39.6% males that refused to participate [58] 18.5% of eligible participants [49] Did not meet inclusion criteria 87% of eligible participants [49] Time commitment 38.9% of active refusals [40] 26.3% of males that refused to participate [58] Invasion of privacy 0.3% of males that refused to participate [58] Medical 15.5% of participants who refused to participate [57] 35.6% of participants unable to attend due to illness, 0.2% of participants had limited medical information [40] 16.9% of males that refused to participate [58] Unable to contact/no response 35.0% of eligible participants [40] 17.4% of eligible participants [41] 40.7% of invited male participants [61] Psychopathology factors 0.8% of males refused to participant in case a medical problem was uncovered [58] Reluctance over medical testing 1.1% of males that refused to participate [58] Religious/philosophical reasons 0.1% of males that refused to participate [58] Third party involvement 62.2% of participants passively refused via a relative, 15.9% of participants passively refused by resident/nursing home [40] 18.4% transferred to another ward or discharged from hospital or research nurse forgot to ask [44] Unknown reason/personal reason 5.2% of males that refused to participate [58] 9.5% of eligible participants in 1968 [45] 28.6% of active refusals [40] 3.2% of eligible participants [41] 3.6% of eligible participants [49]

Proportion of participants who were non-completers/Further suggestions for improvements by completing participants
Appointment non-attendance 3.2% of non-completers [41] 24.7% missed at least one visit by end of study (12 months) [52] Quantitative data-in person visits were difficult to attend due to the distance of the centre [53] Communication Qualitative data-better coordination of communication for study results to participants [38,49] Qualitative data-increased personalisation would increase engagement like a personal question the participants could contemplate over the next week [49] Qualitative data-increase of data sharing between research team, treatment therapist and each participant would have increased engagement and data tracking over the period [49] Education Qualitative data-Increasing the education around the condition that is the focus of the trial [38] Medical 12.7% non-completers [57] 4.1% of non-completers had a child that had an additional diagnosis [51] Qualitative data-state of the participants personal health and the nature of the intervention may affect future enrolment [38] cohort study, non-participation was linked to fathers being born outside the country where the study resided or having lower education [51].Interestingly, Ullman et al., explored factors related to types of study responders; (non-responders, reluctant responders and responders) in an ongoing longitudinal study.Findings demonstrated that males who considered themselves more attractive or having better relationships with others were more likely to respond, while those that felt worse about their own sense of self required more incentives and reminders in order to take part in the study [56].

Facilitators to recruitment and retention of male participants
While many of the facilitators listed in Table 5 would be suitable for either sex, using a male-centric approach would likely prove particularly useful.Study advertising on mainstream media or medical press was used as a method to establish study credibility [42,57].Several studies maintained contact with their participants throughout the study [42,57].In one study, a yearly letter, written by the principal investigator, was sent to participants [42], other studies sent out a study newsletter [36] or monthly emails with health and nutritional scientific information [43].Two studies sent an annual holiday letter [51] and in another participants received birthday and Christmas cards [61].These methods were thought to pique participant interest and motivate them to participate in study activities.Interestingly, Griffith Filipo et al., used humour with participants through the use of humourous GIFs sent to participants following study visits and found these to be a facilitating factor [49].
Other motivational techniques for participation included increasing study accessibility and minimising interference with participants day-to-day activities.For example, one of the studies was designed to ensure that examinations only took a couple of hours [36].Another study was designed to be exclusively online which was a determining factor for participation in 46.45% of the sample [43].In a trial where participants had to answer SMS messages, participants were able to choose the amount, timing, and frequency of texts they received, with the ability to change these options throughout the study course [44].Two studies planned with employers to pay participants regular wages or give leave without penalty while they participated in the study [43].Finally, one study reported that additional interventions were implemented for people that struggled to adhere to the required activities [55].
Incentives were successful in study participation.Six studies gave monetary incentives [46-49, 56, 57].Other studies gave participants small gifts such as membership cards, certificates of completion, pens, tee shirts, mugs, etc. [56,57].In an intervention study where participants had to consume specific products, these products were provided freely for participants [57].A few health interventions offered free medical screenings [43,46,57].Participants in the Rose et al., were notified of the outcome of their medical examinations and alerted if anything was abnormal, which in some cases prevented life-threatening issues [36].To minimise attrition, participant reminders to complete questionnaires or arrange appointments in multiple studies [40,42,46,52,56,57,61].One study found that when participants were contacted to assess reasons for refusal this prompted some to change their minds and participate in the study [40,42,46,52,56,57,61].
One aspect that was associated with male participant retention were the perceived health benefits gained from

Psychopathology factors
Paranoid factor had an elevated but non-significant risk for early drop out (26.9%) Dysphoric Borderline factor put a significant risk for late dropout (15.9%) [50] Qualitative data-side effects from medications for mental health or chronic pain were issues in completing the program.Social anxiety of talking openly to other participants also prohibited some participants interaction [53] Third party involvement 6.8% of non-completers -due to family issues [51] Qualitative data-consider withdrawing when family was sick [38] 4.2% of non-completers-Work and family responsibilities [48] Financial Hardship Qualitative data-participants across all treatment groups found recommendations of what to eat and how to exercise cost prohibitive [53] Qualitative data-providing monetary incentives [38] Technical Issues Qualitative data-difficulties in troubleshooting web-based program after logging in as well as printing physical activity log [53] participating in the study [36,41,57].One study specifically highlighted its participants expressed satisfaction of being part of a "health elite", which was associated with high retention rates [36].Another beneficial factor was the idea that their involvement in the study aided research in the field of nutrition (22.24%) and advanced public health (61.37%) [43].More recently this has been shown to be true during the COVID-19 pandemic where males participated in high rates [36,41,57].Méjean et al., reported that 67.02% of participants expressed satisfaction that the study was funded exclusively by public sources which was perceived as unbiased, and this was particularly well received by male participants [43].
Finally, in an attempt to motivate male participants, Rose et al., enlisted participants' wives in the study, which was found to have positive outcomes in retention [36].
The greatest challenge for data extraction for this review was the way in which authors report these figures in their studies.Many studies report on overall recruitment, retention and barriers but few studies clearly incorporated sex-specific findings.Interests, and drivers for behaviour are unique between sexes and therefore it is important that research projects report separated male and female specific findings.The most beneficial studies reviewed gave recruitment success with each approach, for example Crichton et al. [57], highlighted what number of participants were recruited from varied strategies including advertising via TV or newspaper, letter of invitation, from the hospital, or a notice in the library.The Ullman et al. [56], study was also clear in highlighting how many approaches they needed to have data returned to them, for example, immediately, after one reminder, or multiple reminders and a financial incentive.Likewise, studies who reported when or how they noticed attrition for their research were incredibly valuable [42,44,46,51,57,61].

Strengths and limitations
The strengths of this systematic review lie in its comprehensive compilation of research data from the past 47 years of male recruitment and retention in longitudinal research studies which has historically posed many issues to researchers [5,[10][11][12][13][14][15][16][17][18][19].To the authors' knowledge at the time of print, there is no other systematic review available on the barriers and facilitators of the recruitment and retention of males in longitudinal research.It has been evident that the barriers and facilitators are not unique to a specific study aim but have been experienced across the diverse range of studies.This systematic review offers a comprehensive list of strategies which have worked with particular populations and strategies which have failed for researchers looking to improve their male recruitment and retention rates and has a particular focus on longitudinal research studies.Primarily it has highlighted that multiple facilitators will be needed when designing longitudinal research inclusive of males, as the barriers to participation are diverse.The most challenging barrier to overcome is how to develop enthusiasm and urgency from men towards health research.Regardless of the purpose of the underlying study, the barriers and facilitators for male participants are relatively consistent.
The exclusion of several population groups limits this study however it was felt that each of these required a detailed separate systematic review to ensure that the unique barriers and facilitators for the recruitment and retention of these communities are clearly articulated.A further limitation is that for papers to be included in this systematic review, they had to specifically mention an issue that detailed barriers/facilitators to recruitment/retention in the title/abstract rather than stating "we recruited" in the full text.Therefore, we acknowledge there it may be possible that some publications that focused on longitudinal studies involving male participants have been missed.In conclusion, this systematic review offers an in-depth look into the barriers and facilitators of the recruitment and retention strategies for males aged 17-59 years old for the past 47 years.It highlights that research teams will need to expend considerable time, expense and diverse approaches to successfully engage and retain male participants into longitudinal studies.
was supported by Mater Health, KMR is supported by the Mater Foundation and Equity Trustees (ANZ QLD Community Foundation, QCF-ANZ Bank Fund, QCF-Thomas George Swallow Trust, The HJ Hinchey Cht Trust).

Fig. 1
Fig. 1 PRISMA diagram depicts the search, screening, eligibility and inclusion results

Table 1
General study characteristics.Studies are listed alphabetically

if identified) City (or State/ Province/ Region), Country Study Aim/s Study Design Clinical field Type of data collected
a ClinicalTrials.govNCT03808142 lists multiple locations in Germany, however locations with principal investigators are not listed b Midwestern states are not detailed

Table 2
Study duration, number of study visits, percentage of male participants of the study, and details of recruitment and retention numbers (n.d indicates that no data was available in the published literature)

Table 4
Barriers to participation and drop-out or refusal rates of participants